A common orthopedic joint prosthesis includes a ball and cup arrangement. For example, hip joints typically comprise a rounded femoral head and an acetabular cup. The rounded femoral head is provided on a stem that is configured to engage the intramedullary cavity of the femor and secure the head on the femor. The rounded femoral head includes a convex surface configured to engage a concave surface on the acetabular cup. The acetabular cup is configured for implantation on the acetabulum of the pelvis. When the rounded femoral head is received within the acetabular cup, a ball and socket joint is provided.
In order to reduce wear between the components of the joint prosthesis, the components are manufactured such that the clearance between the bearing surfaces is minimized. The term “clearance” is often used in reference to a “diametral clearance” of the joint prosthesis. The diametral clearance between bearing surfaces is generally considered to be the difference in the diameter defining the bearing surface of the ball and the diameter defining the bearing surface of the cup.
While minimal diametral clearance between the bearing surfaces is desired, at least two factors limit the reduction of clearances. First, manufacturing tolerances generally limit the extent to which clearances may be reduced. For example, for diametral clearances below the 15-30 micron range, it has been observed that imperfect formation of the femoral head and the acetabular cup contributes to local interferences and small deformations that result in wear.
Second, acetabular cup deformation during implantation into the acetabulum also limits the degree to which clearances may be reduced in a hip joint prosthesis. This deformation generally occurs near the equatorial lip of the acetabular cup. For substantially spherical cup and head arrangements, reduction in clearances near the pole of the head also means reduction in clearances near the equatorial lip. In other words, when the head and the cup of a hip prosthesis are substantially spherical, the small clearances near the pole of the head are also found in the region near the equatorial lip of the cup. Thus, when cup deformation occurs near the equatorial lip in a low clearance spherical design, interference is likely to occur between the equatorial lip of the cup and the ball.
One way to reduce clearance complications resulting from acetabular cup deformation is to provide a conformal region having a small clearance near the center of the primary articulation area of the femoral head, and a peripheral region surrounding the conformal region, wherein the peripheral region has a significantly greater clearance than the conformal region, including a significantly greater clearance near the lip of the cup. With this arrangement, deformations near the equatorial lip of the acetabular cup are less likely to result in obstruction with the femoral head because of the increased clearance near the equatorial lip. Although several of these arrangements have been provided in the past, they have not provided optimal solutions. In particular, many of these arrangements include peripheral regions surrounding the conformal region where the clearances in these peripheral regions quickly diverge from the relatively small clearances in the conformal zone. However, when the clearance in the peripheral region is too great, significant wear may result.
Accordingly, what is needed is a joint prosthesis configured to avoid interference between the ball and cup even if the equatorial region of the cup is deformed during implantation. It would also be advantageous if the clearance between the ball and cup could remain relatively low even in a peripheral region surrounding the conformal region.